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Let History Be Our Guide?

posted Jul 23, 2013, 7:12 PM by Anthony Pisano
This week we chose not to highlight Comet ISON, but instead a completely different comet: C/1680 V1 (Kirch). Are we already out of ideas, and bored with Comet ISON? Of course not! Read on... 

Discovered in 1680 (hence the C/1680 designation), comet C/1680 V1 (Kirch), was first spotted by German astronomer Gottfried Kirch in early November, 1680, and holds the accolade of being the first comet to be discovered via a telescope! It also seems to hold the accolade as one of the most aliased comets in history, going also by the names of "The Great Comet of 1680", "Kirch's Comet", and "Newton's Comet"! (The latter is rather misrepresentative, as Newton himself played no part in the discovery, but he did use its orbit to demonstrate his new laws of orbital mechanics when they were published in the famous Principia several years later.) Just a couple of weeks following discovery, C/1680 V1 passed just 0.42AU (~62-million kilometers, or ~39-million miles) from Earth, and just a couple of weeks after that, on Dec 18, 1680, grazed a mere 0.006AU (~900,000km, 550,000miles) from the Sun -- not that far above the solar surface, and reputedly visible during broad daylight! As it raced away from the Sun, it peaked in brightness by the end of the year with a spectacularly long and thin arcing tail that spanned much of the nighttime skies before finally receding from view in early 1681. Sounds great, but what's the link to Comet ISON? Technically there isn't one... but for a while we really thought there was, and despite there not being a link, Kirch's Comet may still hold valuable clues to Comet ISON's fate... 

If we take a glance at the orbital elements for both Comets ISON and Kirch, we see startling similarities. Both comets approach to within ~0.4AU of Earth, and the perihelion distance (closest approach to the Sun) is 0.006AU for Comet Kirch versus 0.012AU for Comet ISON. The so-called "longitude of the ascending node" is 277-degrees for Kirch and 295-degrees for ISON. The "argument of perihelion" is 351-degrees versus 360-degrees, and orbital inclination 61-degrees versus 60-degrees for Kirch and ISON, respectively. These numerical values define the path through space on which these comets travel, and their differences of just a few degrees here and there are not necessarily as big as they sound. In astronomy, we use these values to "link" comets together and determine relationships between them, or determine if we're just seeing the same comet on a new revolution around the Sun, and when we see so many values that are so similar, we sit up and take notice. 

So when we saw the orbital parameters for Comet ISON, we immediately thought that maybe the two comets were related. We knew that they were not the same comet, because Kirch's Comet is periodic, with an orbital period on the order of ten-thousand years, but the possibility certainly existed that these two objects were once part of the same object that fragmented into small pieces some time in the very distant past. We see this all the time with Kreutz Sungrazing comets, so we know it happens. Comet C/2011 W3 (Lovejoy), for example, is strongly believed to be a consequence of the fragmentation of the "Great Comet of 1106", and itself a fragment of a much earlier near-Sun fragmentation even centuries before that. Thus we began to look very closely at ISON's orbit to determine if it was indeed a relative of the Kirch's Comet, with our focus primarily on the orbital parameter of "eccentricity". This value tells us whether the comet's orbit is an ellipse, or a parabola. That is, is it following a closed loop orbit and thus periodic (and gravitationally bound to our Sun), or is it on an "open" path, and thus comes in to the solar system only once and leaves never to be seen again? 

For a while the situation was perched on a knife-edge, but with enough observational data we eventually came to the conclusion that Comet ISON is in fact a dynamically new comet, fresh in from the Oort Cloud -- our solar system's vast and distant reservoir of icy bodies. So again you may rightly question the relevance of C/1680 V1 (Kirch) to C/2012 S1 (ISON). It might not be much, but it is this: Comet Kirch followed a remarkably similar orbit through space at a remarkably similar time of the year, albeit 333-years before ISON, and approached a remarkably similar distance to Earth and a roughly similar distance from the Sun at perihelion (they were both Sungrazers). So while we may not be comparing apples to apples here, it could be that Kirch's comet is one of the only indicators we have of how Comet ISON might look come November/December. Admittedly we don't know if they are of similar size, and C/1680 certainly approached closer to the Sun, but ISON has the wild-card factor that comes with being fresh from the Oort Cloud! As we have already posted on this site, we have no "fresh-Oort-Sungrazer" example that we know of to which we can compare ISON. So the best we can do is look to other Sungrazers, and for the closest examples we can find, and see how they hold up. 

If Comet Kirch is anything to go by, astronomers are looking at a very happy end to the year!